Semiconductor on insulator (SOI) technology is becoming of increasing importance in the field in integrated circuits. SOI technology deals with the formation of transistors in a layer of semiconductor material which overlies an insulating layer on a substrate. A common embodiment of SOI structures is a single crystal layer of silicon which overlies a layer of silicon dioxide.
A prior art semiconductor over insulator ("SOI") transistor includes a semiconductor mesa overlying an insulating layer. The semiconductor mesa has a source region at one end and a drain region at the other end thereof. A body region, body node or channel is disposed between the source and drain regions. Both the source and drain regions are of the same conductivity type and are of opposite conductivity type to that of the body region. For example, the source and drain regions are of an n-type material while body region is of a p-type material. It is to be understood that these types of materials may be reversed such that the source and drain regions are of a p-type material while the body node is of an n-type material. The source and drain regions are of a relatively high dopant concentration level, while the body region is of a relatively low dopant concentration level.
Semiconductor over insulator transistors can be used in many semiconductor device applications, for example, memory devices, microprocessors, controllers, and logic arrays. When the semiconductor over insulator transistors are considered for use in an application, the specific application may require different transistors. Some of the transistors may need long channels while others of the transistors need short channels. Also, there will be some variation of channel length due to process variations. It is important to have long and short channel transistors which have consistent threshold voltages with each other, as device dimensions decrease. It is also important to reduce the different effects of a floating body, in the long and short channel transistors, as much as possible when device dimensions decrease.
Prior art semiconductor on insulator transistors exhibit widely varying threshold voltage characteristics and floating gate effects between long and short channel devices. This is particularly true when the short channel length is reduced below approximately 0.3 .mu.m.